Electromagnetic surface locks for doors are well known and are well exemplified by FIG. 1 of the U.S. Pat. No. 4,652,028 which shows an electromagnetic mounted to the soffit of a doorway and a magnetically attractive armature mounted to the door. When the door is closed and the electromagnet is energized it will exert a substantial holding force on the magnet and lock the door.
The armature is mounted to the door by means of a mounting screw or similar fastening device and will also include guide pins extending into the door to keep the armature from being rotated and maintain it aligned with the electromagnet. It has been common practice to pass the mounting screw or bolt through the armature into the door and also to countersink a large diameter to receive the head of the bolt so that the head of the bolt will not interfere with surface contact of the armature with the electromagnet.
The counter sinking or provision of the recess for the head of the bolt and the passage of the shank of the bolt through the armature requires removal of magnetic material from the armature. This requirement decreases the rigidity of the armature and weakens the armature in the central portion thereof. This decrease in the rigidity will greatly affect the magnetic holding force.
Accordingly, the present invention provides a new and improved armature construction for an electromagnetic surface lock which increases the contact area of the armature with the electromagnet and provides increased rigidity which substantially increases the holding force.